The Focal Mechanism Solutions of the M_S 8.0 Wenchuan Earthquake in Sichuan on May 12,2008 and Some of Its Aftershocks

The focal mechanism solutions of the Wenchuan earthquake （Ms8.0） of May 12, 2008 and some of its aftershocks occurring up to December I0, 2008 are determined with lower semisphere of equal-projection and first motion sign data of P waves from regional and distant stations. The focal mechanism solutions of the Ms8.0 Wenchuan earthquake are： Nodal plane I：strike 5°, dip angle 48°, slip angle 39°; Nodal plane II： strike 247°, dip angle 62°, slip angle 131°; P axis azimuth 309°, plunge 8°, T axis azimuth 208°, plunge 54°, B axis azimuth 44°, plunge 35% Combining geological tectonics and spatial distribution of aftershocks, nodal plane II can be identified as a seismogenic fault. According to focal mechanism solutions, the fault activity that triggered the huge earthquake is reverse thrusting. The main rupture surface is S67°W, basically identical to the fault strike on which the earthquake occurred. The main compression stress P axis is N51°W, which is basically the same as the direction of the regional tectonic stress field. According to the results of focal mechanism solutions of aftershocks, the aftershocks occurring in the southern and northern sections of the Longmenshan fault zone have predominant orientations and are obviously different. For the main shock and the early aftershocks occurring on the southern section of the Longmenshan fault, the rupturing is mainly characterized by reverse-dip slip with some strike-slip, and over time, the aftershocks migrated towards the northern section. The rupturing in the source is mainly characterized by strike-slip with some reverse-dip slips. The stress field is controlled by the main shock stress field in the southern section of the Longmenshan tectonic zone, while it is controlled by the main shock stress field and regional stress field in the northern section of the Longmenshan tectonic zone.